Biogeography of benthic macroinvertebrates in estuaries along the Gulf of Mexico and western Atlantic coasts

The community composition of benthic macroinvertebrates from 870 estuarine sites was examined in order to either confirm or challenge established boundaries of biogeographical provinces along the Gulf of Mexico and western Atlantic coasts of the United States. The objective was two-fold: (1) to demarcate boundaries that separate dissimilar fauna in the Gulf of Mexico, and (2) to compare the Gulf of Mexico biogeographically with other well-known provinces. We segmented the coastline into grid cells with dimensions of 1 degree latitude and 2-4 degree longitude. Using the descriptive techniques of cluster analysis and nonmetric multidimensional scaling, we determined the similarities in benthic community composition between sites within grid cells in the Gulf of Mexico and compared the biotic ordinations to natural habitat characteristics such as salinity, sediment type, and depth. We then evaluated the overall community composition within each grid cell in the Gulf of Mexico and established whether or not similarities existed between adjacent grid cells. In this manner, we confirmed that an east-west gradient existed in estuarine benthic community composition along the Gulf of Mexico coast. This information was combined with our previous work in the western Atlantic coast to discern biogeographical provinces. Based on cluster analyses and an analysis of endemic benthic species the following provinces are proposed: (1) the Virginian province, from Cape Cod, Massachusetts to Wilmington, North Carolina, (2) the Gulf of Mexico, from Rio Grande, Texas to Cape Romano, Florida, and (3) south Florida, south of latitude 26 degree N. The region encompassing South Carolina, Georgia and northern Florida represents a transitional area between temperate and tropical provinces.